[Downloads] Prevent dangerous files from being opened automatically.

chrome/browser/download/download_extensions.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_DOWNLOAD_DOWNLOAD_EXTENSIONS_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_EXTENSIONS_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
 
 namespace download_util {
 
 enum DownloadDangerLevel {
+  // Safe. Or at least not known to be dangerous. Safe to download and open,
+  // even if the download was accidental.
   NOT_DANGEROUS,
+
+  // Require confirmation before downloading. An additional user gesture may not
+  // be required if the download was from a familiar site and the download was
+  // initiated via a user action.
   ALLOW_ON_USER_GESTURE,
+
+  // Always require confirmation when downloading.
   DANGEROUS
 };
 
 // Determine the download danger level of a file.
 DownloadDangerLevel GetFileDangerLevel(const base::FilePath& path);
 
+// Returns true if the file specified by |path| is allowed to open
+// automatically.
+//
+// Not all downloads are initiated with the consent of the user. Even when the
+// user consents, the file written to disk may differ from the users'
+// expectations. I.e. a malicious website could drop a nefarious download
+// possibly by click jacking, or by serving a file that is different from what
+// the user intended to download.
+//
+// Any prompting done in order to validate a dangerous download is a speed bump
+// rather than a security measure. The user likely doesn't have the information
+// necessary to evaluate the safety of a downloaded file. In addition, downloads
+// with a danger type of ALLOW_ON_USER_GESTURE might not prompt at all. So
+// Chrome forces the user to manually open some file types by preventing them
+// from being opened automatically. See https://crbug.com/461858
+//
+// See DownloadAutoOpenHint for details on the criteria used to disallow
+// automatic opening for a file type.
+bool IsAllowedToOpenAutomatically(const base::FilePath& path);
+
 // Tests if we think the server means for this mime_type to be executable.
 bool IsExecutableMimeType(const std::string& mime_type);
 
 }  // namespace download_util
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_EXTENSIONS_H_